1. Field of the Invention
The invention relates to a ceramic heater suitable for heating semiconductors or the like.
2. Related Art Statement
In a system for producing semiconductors, a ceramic heater is applied for heating a wafer so as to deposit a semiconductor thin film on the wafer from gaseous raw materials such as silane gas by means of thermal CVD or the like. In such ceramic heaters, it is required to make the temperature of the heating face and the semiconductor wafer mounted thereon uniform at a high precision.
Several techniques for reducing temperature distribution on the heating (mounting) face of the ceramic heater are known. For example, a so-called two-zone heater is known as such a technique. Such a two-zone heater has a ceramic substrate and inner and outer resistance heat generators made of a metal having a high melting point embedded in the substrate. Separate power supply terminals are connected to the respective heat generators so that electric power may be applied independently on the respective generators. Heat generated from the inner and outer heat generators can be thus independently controlled.
Japanese patent publication 2001-52,843A discloses a heater having a ceramic substrate having nine zone heating resistances embedded therein, and a through hole is formed in the substrate.
When a ceramic heater is used as a susceptor for mounting a semiconductor, various kinds of holes may be provided in the substrate of the heater. Such holes include holes for inserting lift pins for lifting a semiconductor, a gas supply hole for supplying back side gas and a hole for inserting a thermocouple. Such holes, especially through holes, formed in the ceramic substrate result in structural defects.
According to the ceramic heater disclosed in Japanese patent publication 2001-52,843A, the distances between terminals and through holes in the second and third zones viewed from the center are very small. Such a design of terminals and through holes results in cold spots in regions surrounding the terminal and through holes, which deteriorate the temperature uniformity. Further, such cold spot results in a large temperature difference between the region surrounding the terminal and hole and the other region. The ceramic substrate may become susceptible to fracture, due to the temperature difference, which shortens the use life when the heater is repeatedly used.